You can certainly increase your mileage with one of these cars. But it also involves some money. I DD my cars. If you take a look, early 80’s Ford trucks got over 20 mpgs and it is also been purported some 472 and 500 cube Cadillacs also got similar mileage. Todays trucks are getting in the teens with FI and a bunch of computers… over 40 years later. Now, there are certainly trade offs. The trucks I referred to were 300 cid 6 cylinders, with SROD overdrive transmissions and 2.73 rear gears. They also had lightened frames, manual windows, no airbags, etc. Still, in over 40 years you would think mileage would have increased.
As noted above, weight certainly plays a part. I recently weighed my truck and it was 2600 pounds - or less than half the weight of a new truck. I recently changed my gearing from 4.10 to 3.73. I certainly noticed an improvement in mileage. After consulting with some of the fine members here, my next upgrades were a dual carb set up (with the B&B carbs) and dual exhaust. There was another mileage improvement - and the engine is much happier! The mileage improvement is said to be from better fuel distribution. I am happy with it, but ultimately, you need to decide how much money do you want to put into it vs how long would it take to recoup that in savings at the pump. Will the mileage savings be worth it to you? How far do you drive?
A simple carb rebuild might be your most cost effective alternative instead of experimenting with different carbs, OR putting out the money once to get it professionally rebuilt with a warranty by a reputable company.